The present invention relates generally to a method for reel-up of a paper web and a reel-up apparatus for carrying out said method.
The present invention is particularly directed to improvements over conventional methods and devices for reel-up of a paper web which include reduction of web breakage due to wrinkling, sliding at the reeling nip, and an improved distribution of hardness throughout the layers of the roll.
The present invention concerns a method for reel-up of a paper web, wherein a roll is formed around a reeling drum, or in another embodiment of the invention on the support of carrier rolls or carrier belts, and wherein a reeling cylinder is used to form a reeling nip for contacting the roll to be produced during passage of the paper web through a reel-up apparatus.
The present invention further provides a reel-up apparatus intended for carrying out the aforementioned method for paper web reel-up or its equivalent. The reel-up apparatus comprises a reeling cylinder or cylinders wherein the cylinder or cylinders are mounted in connection with the frame construction of the reel-up apparatus and having frame construction supporting members fitted thereon to provide support for the reeling drum around which the roll to be produced is wound as the roll makes its passage through a reeling nip which is formed between the roll and a reeling cylinder.
A Pope-type reel-up is commonly used for the reel-up of a paper web during its passage, e.g., out of a paper machine, coating a machine, supercalender, or printing machine. The web is reeled onto a drum to form a roll which is pressed to form a roll which is pressed against the Pope or reeling cylinder, over which the web runs within a certain sector of said cylinder and said cylinder is rotated at the circumferential speed corresponding to the desired web speed. Before passage of the roll through the reel-up is completed, a new drum having been accelerated to the desired running speed can be brought into nip contact with the Pope cylinder so that the new drum also obtains a speed corresponding exactly to the circumferential speed. As soon as the paper roll has reached the desired diameter, it is displaced apart from the Pope cylinder, thereby gradually slowing down its speed of rotation resulting in the formation of a web-loop or open draw between the new reeling drum and the completed roll. Said web-loop is guided, e.g., by means of a compressed-air jet, to be wound around the new reeling drum, whereby it is separated from the completed roll either by a tearing action or in another conventional arrangement it may be cut-off by means of air jets.
At the highest operating speeds, and in particular when LWC and SC-paper qualities are reeled, an important drawback of Pope-type reel-ups for paper machines and the equivalent is the formation of wrinkles which further results in web breakage at the innermost layers of the rolls. The proportion of web breakage occurring may be as much as 2 to 3 percent which is costly and uneconomical. Variations in hardness occurring in the innermost layers of a paper roll are considered to be the main reason for the formation of wrinkles, so called "bugs" in the innermost layers of a paper roll. Variations in hardness are caused largely due to uncontrollable variations in the line pressure formed between the paper roll and the Pope cylinder as well as variations in the tension of the paper web as it is reeled, which will be described in greater detail later.
The distribution of hardness, or radial variation in density, in a paper roll produced by means of a Pope-type reel-up for paper machines is affected mainly by two factors. The first factor is the line pressure created between the reeling cylinder and the paper roll, and the second factor is the tension of the paper web as it is reeled, i.e. for example, the distribution of hardness is affected by the difference in speed between operation of the press section and operation of the reel-up.
It is accordingly an object of the present invention to try to eliminate these drawbacks by controlling the line pressure placed on the reeling nip. Distribution of hardness that is deficient or distribution of hardness that includes portions of discontinuity causes web breakage in subsequent stages of the paper's treatment, or web run and in the worst case tears and folds are formed in the paper web near the bottom of the roll at its innermost layers.
It is a further object of the present invention to eliminate the drawbacks described above, at least in part.
Another drawback in a conventional reel-up is the occurrence of sliding of the paper web as it is rolled against the face of the reeling cylinder. Several factors affect the amount of sliding that occurs including tension of the paper web in the area of the reeling cylinder, the surface pressure resulting from said tension against the face of the reeling cylinder, and line pressure that is produced by the primary and secondary reeling forks on the growing paper roll which is placed in nip contact against the reeling cylinder. Furthermore, the occurrence of sliding is affected by the surface properties of both the paper web which is to be reeled and the surface properties of the reeling cylinders, i.e. by the friction coefficient created between opposing surfaces as they face each other, which is also affected by the moisture content of the paper.
With increased tension of the paper web, the tendency of sliding to slide is reduced because surface pressure of the paper against the reeling cylinder is elevated and frictional forces are increased. An upper tolerance for an increase in tension of the paper web is regulated by tensile strength of the paper so that an increased frequency of web breakage does not occur.
Maintaining line pressure between the forming roll and the reeling cylinder at a sufficiently high and uniform level is achieved with increased difficulty due to the fact that the reeling begins on primary forks placed in their uppermost positions and is continued at a later stage of web passage on secondary forks. The primary forks bring the roll into downwardly inclined contact with the face of the reeling cylinder whereby the roll receives its initial rotating power from the circumferential speed of the reeling cylinder. As the roll becomes larger and the primary forks are gradually placed in their lowest positions, the line pressure between the roll and the reeling cylinder is reduced evenly and steeply, depending on the reduction of the power component resulting from the force of gravity as the position of the growing roll is changed in relation to the reeling cylinder.
Difficulties occur in controlling line pressure between the roll and the reeling cylinder when the growing roll is shifted from primary forks onto secondary forks. In practice, significant variations do occur in line pressure permitting instances of sliding of the paper web on the face of the reeling cylinder, thus resulting in occasional wrinkles in said paper roll in its initial stage of reeling.
Additional occurrences of sliding and wrinkling are a result of varying surface pressure of the paper roll caused by variations in paper tension as the roll is shifted from the primary forks onto the secondary forks.
In the prior art, means of avoiding the problem of sliding and the additional difficulties manifested as a consequence thereof, which are described above, include setting paper or web tension at its upper tolerance as regulated by a differential in speed between the reeling cylinder and the nearest preceding operating device comprised by the reel-up apparatus. As previously stated, limiting factors consist of an increased susceptibility toward web breakage and deterioration of paper quality properties, e.g. tensile strength of the paper is reduced and elongation is also reduced.
As is well known in the art, a second means employed to increase the line pressure between the growing roll and the reeling cylinder to its maximum tolerance makes use of an excessively high loading force applied at the secondary support forks, in particular, to press the roll against the reeling cylinder.
One drawback of this procedure is deterioration of paper qualities such as tensile strength of the paper and elongation which are reduced as was the case when the preceding prior art means for increasing line pressure is employed. A second drawback in the instant case concerns the production of a roll hardness which increases toward the outermost layers of the roll because those layers become packed with an excessive tightness.